Saudi Arabia has a king, Britain has a queen. That's two nations, then, still clinging to medieval practices in the 21st century. Both are prominent on a shrinking list of monarchies actually relevant to their peoples, but that, as even a republican must admit, is where the similarities end. Their tastes in regrettable medievalisms are certainly not comparable.

Back in the actual Middle Ages, nevertheless, heads of ruling houses were often given to addressing counterparts as "cousin". The Queen and King Abdullah of the House of Saud may take different attitudes to their job descriptions, but anyone unfamiliar with protocol could have mistaken yesterday's scenes in London for a family reunion. As in: "Lovely to see you again. It's been too long. How are the kids? You really must stay for dinner."

Few of us are quite so ignorant of the British version of protocol. If the Queen is wheeled out - or wheeled down the Mall in a carriage while no-one cheers - that's because she is doing her government's bidding. It's what they term a state occasion. It means that someone the government regards as very important indeed has come to town.

We've seen them come and go: Chinese oligarchs, east European dictators, goofy Americans. As a rule, hardy souls protest, generally from behind a thick line of police and security, while the Queen smiles and (no doubt) sucks her teeth. It doesn't make anyone proud to be British, but that's the national interest for you: pragmatism, practical politics, necessity.

At such times, some people question the claim. Why are we putting up with disreputable creep A?

Why are we welcoming clownish thug B? Governments often have half-convincing answers. This time around, the responses have been both defiant and half-hearted, if you can imagine such a thing. King Abdullah's regime is a very hard sell. And that's before the real question has been put. Why is Britain quite so devoted to Saudi Arabia?

The quick answer is money: trade, lots of it, even when the trade happens to be corrupt. The second answer, a favourite of Tony Blair's, is "intelligence co-operation", crucial to the war on international terrorism. The third answer, a familiar one, is oil. The final answer, in which you can hear echoes of the Arabist traditions of the Foreign Office, is "influence".

For each of those there is a response. First, while it is true that the United Kingdom exports £5bn worth of goods and services to Saudi each year, we also sell the regime a great many weapons for wars that are never fought. Does our trade in arms with a government that will never pass as democratic speak well of us? Or does our eagerness to abandon a Serious Fraud Office investigation into the sales methods of BAE Systems - because the Saudis demanded it - call for celebration?

The plug was pulled on that SFO inquiry - into, among other things, $1bn allegedly supplied to a Saudi prince - just as the sale of yet another fleet of jets was at stake. The inquiry is dead; the deal for 72 Eurofighters (to be supplied by BAE) was concluded last month. "Britain" will have earned a minimum of £5bn, and probably much more, while we remember Tony Blair's assurances. The SFO inquiry had to be halted, he said, because it threatened "intelligence co-operation". No mention of jets or the rule of law.

Consider co-operation, instead. King Abdullah comes to Britain whispering the claim that dire Saudi warnings of terrorist threats to this country were ignored. He hints that lives might have been saved on July 7, 2005. A parliamentary committee examined the intelligence involved last year and found it "not credible". So what was the king talking about? What was Blair talking about?

As for the quality of co-operation overall, you need only recall common knowledge. Osama bin Laden is a member of a rich Saudi family. Fifteen of the 19 bombers on 9/11 were Saudis. Many of the foreign insurgents in Iraq are Saudis. Above all, the variant Islamic creed embraced by the Taliban and the fundamentalist madrassas in Pakistan is shared by many Saudis. The "struggle" is also funded by some of them. With the oil price well above $90 a barrel, a great deal of funding can be procured.

This isn't news. By many accounts, Saudi Arabia is the spiritual home of international jihad. The cultists may hate King Abdullah as much as they hate the west, but that scarcely guarantees the regime's reliability as a partner. (Nor does it explain the utility of 72 Eurofighters.) The United States has complained often enough about official Saudi co-operation. Britain is less blunt, shall we say.

Because of oil? We still import a lot of the stuff from Saudi, but less than before, thanks to alternative (albeit precarious) sources. The regime has long wielded its petro-chemical weapon across the world, yet Britain seems more susceptible than most to persuasion. Perhaps, as in the SFO affair, that is what is truly meant by "influence". It exists, but not to our benefit. So Abdullah arrives with, on our government's part, no questions asked.

The problem for our politicians, however, is that the actual, worthwhile, substantive questions are many. Too many to stomach, you might have thought.

State torture. That would be one. It is traditional and commonplace among the police and security forces. Then come ritual public executions and mutilations. The regime deplores any mention of those: why would that be? Possibly because of the absence of anything resembling due legal process. As a few Britons have discovered to their cost, you would not want to enter what passes for a Saudi court.

Religious freedom does not exist. Freedom for women and minorities does not exist. Foreigners, especially decadent allies, are controlled and spied upon. All this while even the lowliest members of the ruling family grow colossally rich and unemployment among young Saudi men - this almost defies belief - is growing fast.

Perhaps we are dealing with simple matters of cultural difference. Perhaps if we tried harder to understand the particular reasons for grotesque corruption it would be easier to watch Gordon Brown give King Abdullah the warmest possible welcome. We could even tell ourselves that there are many lousy regimes in the world, but that we must "reach out", as Mr Brown has famously done recently towards Zimbabwe and Burma.

The corruption problem would remain, however. Not for the Saudis, as such. We could, as so often, call it their problem and decline to interfere. But what the state visit signifies is how all that lovely trade, and all those crucial fighter deals, have corrupted Britain itself. The craven decision to shut down the SFO investigation was a small sign, but a sure one. Governments must often get their hands dirty in the national interest, it is true. So how dirty is too dirty?

Call this an extreme, even fanciful inquiry. If Britain is again subjected to some filthy terrorist attack on behalf of a Saudi-inspired ideology, and if it turns out that there is an actual, physical Saudi connection, what will we do? Ask the right questions, just for a change? Or merely ask for our shiny Eurofighters back?